As feature sizes on integrated circuits approach the minimum size possible with visible or ultraviolet wavelengths optics, other technologies are considered for circuit manufacturing techniques. X-ray proximity lithography is one alternative technique which has been developed, and while its use in circuit production with minimum linewidth 250 nanometers (nm) seems likely, blurring through diffraction at practical mask-to-wafer separation distances complicates extension of the technique to significantly finer line widths. Recently, X-ray projection lithography has begun to be considered. J. E. Bjorkholm et al, in Jour. Vac. Sci. Technol., vol. B8 (1990) pp. 1509-1513, have demonstrated the printing of 50 nm line width features using a 20:1 reduction system based on near-normal incidence optics, coated with multilayers for good reflectivity at .lambda.=14 nm wavelength. Along with other developments, this has led to considerable activity aimed at the development of X-ray projection lithography systems capable of printing features of size 100 nm or finer over a large field with high throughput for use in circuit production.
Most of the optical systems discussed in the literature for use in X-ray projection lithography have been based on the use of multilayer-coated, near-normal incidence optics. At present, multilayer-coated optics are able to deliver good normal incidence reflectivity only at relatively long wavelengths (.lambda.&gt;10 nm), where X-ray penetration in photoresist materials is low and the contrast of likely contaminants is high compared to the .lambda..apprxeq.1 nm wavelengths used for X-ray proximity lithography. Further, while 1:1 systems using only flat and spherical optics have been proposed which would have curved image fields, T. E. Jewell et al, in Jour. Vac. Sci Technol., vol. B8 (1990) pp. 1519-1523, have found that at least 4-aspherical optics are required for a 20:1 reduction system with the required field and resolution. The figure tolerances of such optics are in the 0.5-1 nm range over a diameter of many cm, which is well beyond current fabrication limits, even for spherical optics, as noted in W. Silfvast, ed., Workshop on High Precision Soft X-ray Optics, Rockville, Md., October 1989. These and other considerations indicate that the challenges involved in the development of X-ray projection lithography by optical reduction are daunting, or even insurmountable, even considering the commercial payoff expected of such systems.
Holographic optics were first proposed over twenty years ago for visible light lithography, by K. A. Stetson, Appl. Phys., Lett., vol. 12 (1967) pp. 362-364, by E. B. Champayne et al, Appl. Optics, vol. 8 (1969) pp. 1879-1885, and by M. J. Beesley et al, Electronics Lett., vol. 4 (1970) pp. 49-50. These methods did not become popular, because better and easier alternatives were developed based on the use of lenses for optical reduction. However, once one considers soft X-ray projection lithography, for which refracting lenses cannot be used, the technology for reduction imaging outlined above involves the enormous technological challenges referred to above, and it becomes worthwhile to consider again the potential contribution of holographic techniques.
What is needed is a technique for optical lithography that: (1) allows definition of integrated circuit feature sizes of the order of 0.25 .mu.m and below; (2) is simple, preferably requiring only one or two optical components; (3) is relatively free of optical aberrations; (4) is relatively easy to fabricate; (5) allows reasonably uniform illumination of the desired image area; and (6) provides some means of dealing with high incoming power loads.